This invention relates to techniques and apparatus for efficient encoding and decoding of communication signals, and in particular to techniques and apparatus for encoding and decoding using code position modulation for use in wireless personal area networks.
Code Position Modulation (CPM) has recently been selected as the modulation format for the IEEE 802.15.4 standard for low-rate wireless personal area networks (WPAN""s). Size, cost, and power consumption are critical parameters for devices within this standard. Consequently, there is a continuing need for simplified encoding/decoding techniques that result in more efficient data transfer across the network.
In code position modulation (CPM), each transmitted symbol is represented by a N-chip pseudo-noise (PN) sequence. k bits of information can be encoded into each symbol by circularly shifting the N-chip sequence to one of 2k positions (where 2k is less than or equal to N).
In the IEEE 802.15.4 standard, the type of PN sequence used to represent each symbol is a xe2x80x9cmaximal-length linear feedback shift register sequencexe2x80x9d or xe2x80x9cm-sequencexe2x80x9d. m-sequences have good auto-correlation properties, making it easy to distinguish phase shifts (different code positions) of the sequence, and they have reasonably good cross-correlation properties.
Prior methods for generating the circularly shifted symbol include the use of a look-up table that contains the N-chip sequences for each of the 2k required shifts. This requires a large amount of memory for large values of k. An alternative uses a circular shift register and a down-counter. The down-counter is loaded with the shift value, and the shift register is shifted once for each count of the down-counter. When the counter reaches zero, the shift register contains the required symbol.
However, memory and down-counters add cost and complexity to the encoder (and to the corresponding decoder). Consequently, there is a need for a simpler, less expensive technique and apparatus for generating CPM symbols.